Transformer providing half-turn secondary windings

ABSTRACT

A transformer is provided with a closed core having a center leg on which a primary winding is disposed and having two outer legs on which one or more secondary windings having an integer number of half turns are disposed. To insure flux equality in the two outer legs, balancing windings may be disposed on the outer legs or, alternatively, the secondary windings having the integer number of half turns may be wound in duplicate and crossconnected in parallel so that an extra half turn of one of these secondary windings passes between the center leg and one of the outer legs and an extra half turn of another of these secondary windings passes between the center leg and the other of the outer legs.

United States Patent [191 [111 3,768,055 Oliver Oct. 23, 1973TRANSFORMER PROVIDING HALF-TURN Primary Examiner-Thomas J. KozmaSECONDARY WINDINGS Att0rneyRoland I. Griffin [75] Inventor: Bernard M.Oliver, Los Altos Hills,

Calif- 57 ABSTRACT Assigneei uewleu'ltackard p y Pal) A transformer isprovided with a closed core having a Alto, Callfcenter leg on which aprimary winding is disposed and [22] Filed: June 23, 1972 having twoouter legs on which one or more secondary windings having an integernumber of half turns are PP 265,952 disposed. To insure flux equality inthe two outer legs, balancing windings may be disposed on the outer legs52 us. (:1 336/73 336/172 336/184 alternatively the secmdary windingshaving the 51 Int. Cl. .j Him 27/28 integer number half urns may be indupli- [58] Field of Search 336/73 174 175 Cate and cmss-mnnected inParallel that 336/182 half turn of one of these secondary windingspasses between the center leg and one of the outer legs and [56]References Cited an extra half turn of another of these secondary wind-UNITED STATES PATENTS ings passes between the center leg and the otherof the outer le s. 3,431,487 3/1969 Savage 336/73 g 5 Claims, 8 DrawingFigures Copper Srrops Folded 0nd Soldered 23\ Solder- Insulation Solder12 PAIENIEBnm 23 ms sum 1 or 41 llllll l igure 2 PAIENIEnnn 23 ms SHEET0F 4 2 m mm a: 2 2.6 uwtwws 960 EO V2235 35m 5300 TRANSFORMER PROVIDINGHALF-TURN SECONDARY WINDINGS BACKGROUND OF THE INVENTION In the use ofelectrical transformers comprising a primary winding and a plurality ofsecondary windings on a core, it is often desirable, in order to providevoltages from the secondary windings that are very close to a desiredratio to each other, to be able to add the voltage corresponding to onehalf-turn of one secondary winding to another secondary winding. This isparticularly true where the voltages desired are comparable to thevoltage per turn; for example, in low voltage applications, whereone-half volt increments are desired from secondary windings producingone volt per turn.

In a standard form of transformer utilizing EI laminations in thetransformer core, a half-turn secondary winding is formed by a wire thatthreads one window of the core but not the other window. Therefore, asingle turn of wire around the center leg of the transformer core,center-tapped to ground, forms two halfturn windings, one on each sideof the center tap. So long as the two half-turn secondary windings areloaded symmetrically, the two secondary voltages and currents remainbalanced. However, when the loading of the two half-turn secondarywindings is asymmetrical, for example, a load on one half-turn secondarywinding but not the other, the leakage reactance of the loaded halfturnsecondary winding is much larger than the leakage reactance of onefull-turn secondary winding, and the regulation of the transformer isvery poor.

The added leakage reactance results from the fact that the current drawnthrough the loaded half-turn secondary winding sets up a counter flux inthe core leg associated with the loaded half-turn secondary winding, andthis flux travels around the core in a path including the other core legassociated with the other half-turn secondary winding and adds to theflux in that other core leg. The voltage across the loaded half-turnsecondary winding tends todecrease while the voltage across the otherhalf-turn secondary winding tends to increase, and the regulation ispoor.

One possible solution to this problem is to create an air gap betweenthe two core halves, each including one outer leg, such that fluxgenerated in one outer leg cannot be added to the flux in the otherouter leg. This essentially separates the transformer into twotransformers with a common primary, either of which can be loaded. Sucha configuration has merit'if the transformer core consists of two pairsof C-cores, but is not practical when utilizing EI lamination typecores.

SUMMARY OF THE INVENTION In the present invention one or moreshort-circuited secondary windings are associated with the two outerlegs of the core in such a manner that the normal magnetizing flux inthe center leg of the core, which flux divides equally between the twoouter legs, induces no emfs around these windings. However, any fluxproduced by the mmf of a current drawn from a half-turn winding doesinduce an emf in these windings, causing current to flow therein such asto oppose the flux. This forces a flux balance in the two outer legs,and a substantial reduction of the large leakage reactance normallyproduced by the aforementioned flux imbalance.

In one embodiment of the present invention, the short-circuitedsecondary winding comprises a first III winding associated with oneouter leg of the transformer core and series connected with a secondwinding associated with the other outer leg of the core. Each of thesesecondary windings passes through its associated window in the core inthe same direction and the same number of times. The two secondarywindings are connected in parallel so that the emfs produced by thenormal balanced magnetizing flux oppose each other, and no circulatingcurrent flows.

In another embodiment of this invention the shorted secondary windingstructure consists of a single turn associated with each core leg, thesingle turns being connected together as before.

DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are front and top views,respectively, of a transformer core having a pair of half-turn windingsthereon.

FIG. 3 is an equivalent circuit diagram of the two half-turn windings ofFIG. 1.

FIG. 4 is a view of the transformer core of FIG. 1 showing a balancingsecondary winding wound thereon in accordance with one embodiment of thepresent invention.

FIG. 5 is an equivalent circuit diagram similar to the circuit of FIG. 3with the secondary of FIG. 4 added thereto.

FIG. 6 is a view ofa transformer core including a balancing secondarywinding structure in accordance with a second embodiment of theinvention.

FIGS. 7(a) and 7(b) are two views of a transformer showing the formationof the secondary windings of FIG. 6 on the core.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2,there is shown a typical transformer core utilizing standard Ellaminations and having a center leg portion 11 on which it is assumedthe primary winding is wound and two outer leg portions, the right leg12 and the left leg 13. A half-turn winding is formed by a wire thatthreads one window of the core but not the other window. For example,wire AB passing through window 14 forms a half-turn winding while wireB"C passing through window 15 forms a second half-turn winding. If thewire ends B and b" are connected together to form a junction B, then afull turn winding ABC is formed. If A is then connected to C, thecircuit will enclose (link) the flux in the center leg of the core.However if A is connected to B, only the flux in the left leg 13 islinked. If point B is grounded, the open circuit voltages to ground atpoints A and C will be -E/2 and E/2, respectively, where E is thevoltage induced in one full-turn winding ABC. Thus, so far asopen-circuit voltages are concerned, both AB and BC are half-turnwindings, and these half-turn winding are added to associated secondarywindings on the core to provide secondary windings each consisting of aninteger number of turns plus one half-turn.

So long as these two half-turn winding AB and BC are loadedsymmetrically, equal currents will be drawn through them, and the fluxproduced in the core by the current in one secondary winding will becounterbalanced by the flux produced in the core by the current in theother secondary winding. However, if the two half-turn windings areloaded asymmetrically, then one of the half-turn windings will draw agreater current than the other, and the flux in one outer core leg willdecrease while that in the other outer core leg will increase. As aresult, voltage will be substracted from the loaded secondary windingand added to the unloaded secondary winding and regulation is poor.

This may be more readily understood by referring to the equivalentcircuit of the secondary winding ABC of FIGS. 1 and 2 shown in FIG. 3.When a load is connected from C to ground, the current flowing from B toC and back to B via ground causes an upward mmf in the right leg 12 ofthe core. The flux in center leg 11 cannot be altered because of thevoltage driven primary winding around the center leg. However, the fluxproduced by the upward mmf can return down the left leg 13. Thus, thehalf-turn winding BC is linked by a permeable ring comprising the topand bottom of the core (a length of 4a) and the two side'legs (a lengthof 2b).

The leakage reactance between A and C is L (lk where L is the inductanceof the secondary with the primary open and k is the couplingcoefficient. If k is, for example 0.995, then the leakage reactance of afull turn is L (lk )-L /100. The leakage inductance, L, added by thehalf-turn winding BC as a result of the ring core is L L (reluctance tonormal flux/reluctance to flux around outside ring) or z L (a b) /(4a2b) Assuming b 2a, then L z 3/8L2,

other side and the regulation is poor.

One possible manner by which the inductance L can be greatly reduced isby separating the core into two halves by introducing an air gap in theplane P of FIG. l.'This separates the transformer into two transformerswith a common primary, and either of the two transformers can be loaded.Although this arrangement has merit if the core consists of two pairs ofC-cores, it is not practical when using EI laminations.

In accordance with one embodiment of the present invention, andreferring to FIG. 4, the leakage inductance L is greatly reduced byproviding a shortcircuited secondary winding configuration comprising afirst winding section 16 about the left leg 13 and a second windingsection 17 about the right leg 12. Each secondary winding section 16 or17 passes through its associated window 14 or in the same direction thesame number of times. As a result, the normal magnetizing flux in thecenter leg 11, dam, which divides equally into the two outer legs 12 and13 induces no emfs around these windings. Any leakage flux, produced bythe mini of a current drawn from a half-turn winding does induce emfscausing currents to flow in these windings which oppose the leakageflux, and the effect is to force a flux balance in the two outer legs.As a result, half-turns may be added to any secondary winding on thecenterleg without incurring a large leakage reactance.

The equivalent circuit diagram of FIG. 5, which is similar to that ofFIG. 3, shows the short-circuited secondary coupled to L; in FIG. 5, Ris the resistance of the balancing winding of FIG. 4 while N is thetotal number of turns in the balancing winding. In this equivalentcircuit, L has been effectively replaced by R/N' The windings of FIG. 4can be prewound single layer solenoids that are soldered together at Xand Y after core assembly. It should be noted that the balancingwindings themselves can be used to give half-turn voltage steps as forexample, between terminals X and Y.

Another embodiment of the present invention is shown in FIG. 6 wherein apair of short-circuited secondaries 21 and 22 are employed. As with theembodiment of FIG. 4, each secondary circuit passes through each windowin the same direction the same number of times (in this illustrationonce). In the equivalent circuit of FIG. 5, R is the half the resistanceof either winding of FIG. 6 while N is the total of turns.

The secondary windings of FIG. 6 may be formed on the core structure asshown in FIGS. 7(a) and 7(b) by first inserting four short strips 23 ofcopper through the two windows 14 and 15, the strips 23 being insulatedfrom the core, the ends of the strips being folded against the sides ofthe core as shown. Thereafter, the shorted secondary windings may beformed-by soldering the ends of the two suitably insulated crossedcopper strips 24 to the associated pair of strap inserts 23 to form thefigure eight secondary windings 21 and 22 on the top and bottom sides ofthe core.

I claim:

1. A transformer comprising a core structure including a central legportion and two outer leg portions with window openings in the corestructure between the central leg portion and the outer leg portions, aprimary winding disposed on the central leg portion, a pair of secondarywindings disposed on the core structure, at least one of these secondarywindings including a halfturn winding comprising a wire passing in onedirection through the window opening between the central leg portion andone of the outer leg portions so that current passing through thehalf-turn winding produces a flux in said one of the outer leg portions,and a shortcircuited winding structure disposed on both outer legportions so that a current is induced in the shortcircuited windingstructure by the flux in said one of the outer leg portions to produce acounterbalancing flux in the other of the outer leg portions, saidshortcircuited winding structure comprising a first plural turn windingdisposed on one of the outer leg portions and a second plural turnwinding'disposed on the other of the outer leg portions, said first andsecond plural turn windings passing through their associated windowopenings in the same direction and the same number of times and havingtheir ends interconnected in series.

2. A transformer comprising a core structure including a central legportion and two outer leg portions with window openings in the corestructure between the central leg portion and the outer leg portions, aprimary winding disposed on the central leg portion, a pair of secondarywindings disposed on the core structures, at least one of thesesecondary windings including a halftum winding comprising a wire passingin one direction through the window opening between the central legportion and one of the outer leg portions so that current passingthrough the half-turn winding produces a flux in said one of the outerleg portions, and a shortcircuited winding structure disposed on bothouter leg portions so that a current is induced in the shortcircuitedwinding structure by the flux in said one of the outer leg portions toproduce a counterbalancing flux in the other of the outer leg portions,said shortcircuited winding structure comprising a pair ofshortcircuited winding circuits each including a pair of windings witheach winding of the pair being coupled to a different one of the outerleg portions and passing through its associated window opening in thesame direction and the same number of times.

3. A transformer comprising a core structure includin g a central legportion and two outer leg portions with window openings in the corestructure between the central leg portion and the outer leg portions, aprimary winding disposed on the central leg portion, a pair of secondarywindings disposed on the core structure, said secondary windingsincluding a wire passing through each window opening and having a tap atits midpoint to provide two half-turn secondary windings, currentpassing through one of these half-turn secondary windings producing aflux in the core structure, and a shortcircuited winding structuredisposed on both outer leg portions so that a current is induced in theshortcircuited winding structure by said flux to produce a flux in thecore structure counter balancing the flux produced therein by thecurrent in said one of the halfturn secondary windings.

4. A transformer as in claim 3 wherein said shortcircuited windingstructure comprises a pair of windings each coupled to a different oneof the outer leg portions, both of these windings being connected inseries and passing through their associated window openings in the samedirection and the same number of times.

5. A transformer as in claim 3 wherein said shortcircuited windingstructure comprises a pair of shortcircuited winding circuits, eachincluding a pair of windings with each winding of the pair being coupledto a different one of the outer leg portions and passing through itsassociated window opening in the same direction and the same number oftimes.

1. A transformer comprising a core structure including a central legportion and two outer leg portions with window openings in the corestructure between the central leg portion and the outer leg portions, aprimary winding disposed on the central leg portion, a pair of secondarywindings disposed on the core structure, at least one of these secondarywindings including a half-turn winding comprising a wire passing in onedirection through the window opening between the central leg portion andone of the outer leg portions so that current passing through thehalf-turn winding produces a flux in said one of the outer leg portions,and a short-circuited winding structure disposed on both outer legportions so that a current is induced in the short-circuited windingstructure by the flux in said one of the outer leg portions to produce acounterbalancing flux in the other of the outer leg portions, saidshort-circuited winding structure comprising a first plural turn windingdisposed on one of the outer leg portions and a second plural turnwinding disposed on the other of the outer leg portions, said first andsecond plural turn windings passing through their associated windowopenings in the same direction and the same number of times and havingtheir ends interconnected in series.
 2. A transformer comprising a corestructure including a central leg portion and two outer leg portionswith window openings in the core structure between the central legportion and the outer leg portions, a primary winding disposed on thecentral leg portion, a pair of secondary windings disposed on the corestructure, at least one of these secondary windings including ahalf-turn winding comprising a wire passing in one direction through thewindow opening between the central leg portion and one of the outer legportions so that current passing through the half-turn winding producesa flux in said one of the outer leg portions, and a short-circuitedwinding structure disposed on both outer leg portions so that a currentis induced in the short-circuited winding structure by the flux in saidone of the outer leg portions to produce a counterbalancing flux in theother of the outer leg portions, said short-circuited winding structurecomprising a pair of short-circuited winding circuits each including apair of windings with each winding of the pair being coupled to adifferent one of the outer leg portions and passing through itsassociated window opening in the same direction and the same number oftimes.
 3. A transformer comprising a core structure including a centralleg portion and two outer leg portions with window openings in the corestructure between the central leg portion and the outer leg portions, aprimary winding disposed on the central leg portion, a pair of secondarywindings disposed on the core structure, said secondary windingsincluding a wire passing through each window opening and having a tap atits midpoint to provide two half-turn secondary windings, currentpassing through one of these half-turn secondary windings producing aflux in The core structure, and a short-circuited winding structuredisposed on both outer leg portions so that a current is induced in theshort-circuited winding structure by said flux to produce a flux in thecore structure counter balancing the flux produced therein by thecurrent in said one of the half-turn secondary windings.
 4. Atransformer as in claim 3 wherein said short-circuited winding structurecomprises a pair of windings each coupled to a different one of theouter leg portions, both of these windings being connected in series andpassing through their associated window openings in the same directionand the same number of times.
 5. A transformer as in claim 3 whereinsaid short-circuited winding structure comprises a pair ofshort-circuited winding circuits, each including a pair of windings witheach winding of the pair being coupled to a different one of the outerleg portions and passing through its associated window opening in thesame direction and the same number of times.